Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Characteristics and source apportionment of water-soluble organic nitrogen (WSON) in PM2.5 in Hong Kong: With focus on amines, urea, and nitroaromatic compounds

Water-soluble organic nitrogen (WSON) is ubiquitous in fine particulate matter (PM2.5) and poses health and environmental risks. However, there is limited knowledge regarding its comprehensive speciation and source-specific contributions. Here, we conducted chemical characterization and source apportionment of WSON in 65 PM2.5 samples collected in Hong Kong during a 1-yr period. Using various mass-spectrometry-based techniques, we quantified 22 nitrogen-containing organic compounds (NOCs), including 17 nitroaromatics (NACs), four amines, and urea. The most abundant amine and NACs were dimethylamine and 4-nitrocatechol, respectively. Two secondary (i.e., secondary formation and secondary nitrate) and five primary sources (i.e., sea salt, fugitive dust, marine vessels, vehicle exhaust, and biomass burning) of WSON and these three categories of NOCs were identified. Throughout the year, secondary sources dominated WSON formation (69.0%), while primary emissions had significant contributions to NACs (77.1%), amines (75.9%), and urea (83.7%). Fugitive dust was the leading source of amines and urea, while biomass burning was the main source of NACs. Our multi-linear regression analysis revealed the significant role of sulfate, NO3, nitrate, liquid water content, and particle pH on WSON formation, highlighting the importance of nighttime NO3 processing and heterogeneous and aqueous-phase formation of NOCs in the Hong Kong atmosphere.

Chemodiversity of organic nitrogen emissions from light-duty gasoline vehicles is governed by engine displacements and driving speed

Organic nitrogen emissions from light-duty gasoline vehicles (LDGVs) is believed to play a pivotal role in atmospheric particulate matter (PM) in urban environments. Here, the characterization of organic nitrogen emitted by LDGVs with varying engine displacements at different speed phases was analyzed using a Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) at molecular level. For the LDGV with small engine displacements, the nitrogen-containing organic (CHON) compounds exhibit higher abundance, molecular weight, oxygen content and aromaticity in the extra-high-speed phase. Conversely, for the LDGV with big engine displacements, more CHON compounds with elevated abundance, molecular weight, oxygen content and aromaticity were observed in the low-speed phase. Our study assumed that the formation of CHON compounds emitted from LDGVs is mainly the oxidation reaction during fuel combustion, so the potential precursor-product pairs related to oxidation process were used to study the degree of combustion reaction. The results show that the highest proportion of oxidation occurs during extra-high-speed phase for LDGV with small engine displacements, and during low-speed phase for LDGV with big engine displacements. These results offer a novel perspective for comprehending the mechanism behind vehicle emissions formation and contribute valuable insights for crafting effective air pollution regulations.

A three-channel thermal dissociation cavity ring-down spectrometer for simultaneous measurement of ambient total peroxy nitrates, total alkyl nitrates, and NO2

A newly constructed thermal dissociation cavity ring-down spectrometer (TD-CRDS) for the simultaneous measurement of ambient total peroxy nitrates (ΣPNs, RO2NO2), total alkyl nitrates (ΣANs, RONO2), and NO2 was presented in this work. ΣPNs and ΣANs were detected as NO2 with the CRDS instrument after thermal dissociation. PNs and ANs completely dissociated at 180 °C and 360 °C, with conversion efficiencies of 96 % and 99 %, respectively. The effects of NO2 and NO on measurement in different temperatures and two types of thermal dissociation inlet (TDI) were further explored. The influence of ambient NO2 and NO on PNs and ANs in the improved TDI (TDI-2) was significantly improved. To further enhance the measurement accuracy, the consistency of the observed NO2 in the three channels was tested, which achieved good agreement. The detection limits of the TD-CRDS instrument for NO2, ΣPNs, and ΣANs were determined as 6.5, 6.8, and 8.6 pptv (10 s, 1σ), respectively. Observations of PNs and ANs were conducted in a suburban site in Hefei, China, from September 2-30, 2021, using the TD-CRDS instrument, and the consecutive time series of PNs and ANs were derived, verifying the capability of the TD-CRDS instrument for continuous field observations of ΣPNs and ΣANs.

Development of an automatic measurement system using atmospheric pressure photoionization ultrahigh-resolution mass spectrometry and application for on-line analysis of particulate matter

On-line chemical characterization of atmospheric particulate matter (PM) with soft ionization technique and ultrahigh-resolution Mass Spectrometry (UHRMS) provides molecular information of organic constituents in real time. Here we describe the development and application of an automatic measurement system that incorporates PM2.5 sampling, thermal desorption, atmospheric pressure photoionization, and UHRMS analysis. Molecular formulas of detected organic compounds were deducted from the accurate (±10 ppm) molecular weights obtained at a mass resolution of 100,000, allowing the identification of small organic compounds in PM2.5. Detection efficiencies of 28 standard compounds were determined and we found a high sensitivity and selectivity towards organic amines with limits of detection below 10 pg. As a proof of principle, PM2.5 samples collected off-line in winter in the urban area of Beijing were analyzed using the Ionization Module and HRMS of the system. The automatic system was then applied to conduct on-line measurements during the summer time at a time resolution of 2 hr. The detected organic compounds comprised mainly CHON and CHN compounds below 350 m/z. Pronounced seasonal variations in elemental composition were observed with shorter carbon backbones and higher O/C ratios in summer than that in winter. This result is consistent with stronger photochemical reactions and thus a higher oxidation state of organics in summer. Diurnal variation in signal intensity of each formula provides crucial information to reveal its source and formation pathway. In summary, the automatic measurement system serves as an important tool for the on-line characterization and identification of organic species in PM2.5.

Understanding the Formation and Growth of New Atmospheric Particles at the Molecular Level through Laboratory Molecular Beam Experiments

Atmospheric new particle formation (NPF), which exerts comprehensive implications for climate, air quality and human health, has received extensive attention. From molecule to cluster is the initial and most important stage of the nucleation process of atmospheric new particles. However, due to the complexity of the nucleation process and limitations of experimental characterization techniques, there is still a great uncertainty in understanding the nucleation mechanism at the molecular level. Laboratory-based molecular beam methods can experimentally implement the generation and growth of typical atmospheric gas-phase nucleation precursors to nanoscale clusters, characterize the key physical and chemical properties of clusters such as structure and composition, and obtain a series of their physicochemical parameters, including association rate coefficients, electron binding energy, pickup cross section and pickup probability and so on. These parameters can quantitatively illustrate the physicochemical properties of the cluster, and evaluate the effect of different gas phase nucleation precursors on the formation and growth of atmospheric new particles. We review the present literatures on atmospheric cluster formation and reaction employing the experimental method of laboratory molecular beam. The experimental apparatuses were classified and summarized from three aspects of cluster generation, growth and detection processes. Focus of this review is on the properties of nucleation clusters involving different precursor molecules of water, sulfuric acid, nitric acid and NxOy, respectively. We hope this review will provide a deep insight for effects of cluster physicochemical properties on nucleation, and reveal the formation and growth mechanism of atmospheric new particle at the molecular level.

Modeling Novel Aqueous Particle and Cloud Chemistry Processes of Biomass Burning Phenols and Their Potential to Form Secondary Organic Aerosols

Phenols emitted from biomass burning contribute significantly to secondary organic aerosol (SOA) formation through the partitioning of semivolatile products formed from gas-phase chemistry and multiphase chemistry in aerosol liquid water and clouds. The aqueous-phase SOA (aqSOA) formed via hydroxyl radical (•OH), singlet molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*), which oxidize dissolved phenols in the aqueous phase, might play a significant role in the evolution of organic aerosol (OA). However, a quantitative and predictive understanding of aqSOA has been challenging. Here, we develop a stand-alone box model to investigate the formation of SOA from gas-phase •OH chemistry and aqSOA formed by the dissolution of phenols followed by their aqueous-phase reactions with •OH, 1O2*, and 3C* in cloud droplets and aerosol liquid water. We investigate four phenolic compounds, i.e., phenol, guaiacol, syringol, and guaiacyl acetone (GA), which represent some of the key potential sources of aqSOA from biomass burning in clouds. For the same initial precursor organic gas that dissolves in aerosol/cloud liquid water and subsequently reacts with aqueous phase oxidants, we predict that the aqSOA formation potential (defined as aqSOA formed per unit dissolved organic gas concentration) of these phenols is higher than that of isoprene-epoxydiol (IEPOX), a well-known aqSOA precursor. Cloud droplets can dissolve a broader range of soluble phenols compared to aqueous aerosols, since the liquid water contents of aerosols are orders of magnitude smaller than cloud droplets. Our simulations suggest that highly soluble and reactive multifunctional phenols like GA would predominantly undergo cloud chemistry within cloud layers, while gas-phase chemistry is likely to be more important for less soluble phenols. But in the absence of clouds, the condensation of low-volatility products from gas-phase oxidation followed by their reversible partitioning to organic aerosols dominates SOA formation, while the SOA formed through aqueous aerosol chemistry increases with relative humidity (RH), approaching 40% of the sum of gas and aqueous aerosol chemistry at 95% RH for GA. Our model developments of biomass-burning phenols and their aqueous chemistry can be readily implemented in regional and global atmospheric chemistry models to investigate the aqueous aerosol and cloud chemistry of biomass-burning organic gases in the atmosphere.

Kinetics of the reaction of OH with methyl nitrate (223-343 K)

Rate coefficients (k4) for the reaction of hydroxyl radicals (OH) with methyl nitrate (CH3ONO2) were measured over the temperature range 232-343 K using pulsed laser photolysis to generate OH and pulsed laser-induced fluorescence to detect it in real-time and under pseudo-first-order conditions. In order to optimize the accuracy of the rate coefficients obtained, the concentration of CH3ONO2 (the reactant in excess) was measured on-line by absorption spectroscopy at 213.86 nm for which the absorption cross-section was also measured (σ213.86 = 1.65 ± 0.09 × 10-18 cm2 molecule-1). The temperature-dependent rate coefficient is described by k4(T) = 7.5 × 10-13 exp[(-1034 ± 40)/T] cm3 molecule-1 s-1 with a room temperature rate coefficient of k4(296 ± 2 K) = (2.32 ± 0.12) × 10-14 cm3 molecule-1 s-1 where the uncertainty includes the statistical error of 2σ and an estimation of the potential systematic bias of 5%. This new dataset helps to consolidate the database for this rate coefficient and to reduce uncertainty in the atmospheric lifetime of CH3ONO2. As part of this study, an approximate rate coefficient for the reaction of H-atoms with CH3ONO2 (k9) was also derived at room temperature: k9(298 K) = (1.68 ± 0.45) × 10-13 cm3 molecule-1 s-1.

Olfaction in the Anthropocene: NO3 negatively affects floral scent and nocturnal pollination

There is growing concern about sensory pollutants affecting ecological communities. Anthropogenically enhanced oxidants [ozone (O3) and nitrate radicals (NO3)] rapidly degrade floral scents, potentially reducing pollinator attraction to flowers. However, the physiological and behavioral impacts on pollinators and plant fitness are unknown. Using a nocturnal flower-moth system, we found that atmospherically relevant concentrations of NO3 eliminate flower visitation by moths, and the reaction of NO3 with a subset of monoterpenes is what reduces the scent's attractiveness. Global atmospheric models of floral scent oxidation reveal that pollinators in certain urban areas may have a reduced ability to perceive and navigate to flowers. These results illustrate the impact of anthropogenic pollutants on an animal's olfactory ability and indicate that such pollutants may be critical regulators of global pollination.

Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.

Decreasing Production and Potential Urban Explosion of Nighttime Nitrate Radicals amid Emission Reduction Efforts

Nighttime oxidation by nitrate (NO3) radicals has important ramifications on nocturnal aerosol formation and hence the climate and human health. Nitrate radicals are produced by the reaction of NO2 and O3. Despite large decreases in anthropogenic emissions of nitrogen oxides (NOx = NO + NO2), a previous study found significant increases in NO3 production (PNO3) from 2014 to 2019 in China, in contrast to decreasing trends in the U.S. and Europe. Using the summer observations from 2014 to 2022, we analyze the interannual variability of nocturnal PNO3 using a systematic framework, in which PNO3 is diagnosed as a function of odd oxygen (Ox = O3 + NO2) and the NO2/O3 ratio. We did not find an increase of PNO3 from 2014 to 2022 in China due to a continuous decrease in the NO2/O3 ratio, although PNO3 is modulated by the variation in Ox. Using in situ observations obtained in Beijing in 2007, we demonstrate the potential for an upsurge resembling an "explosion" in urban nighttime NO3 radicals amid emission reduction efforts.

Dissecting the contributions of organic nitrogen aerosols to global atmospheric nitrogen deposition and implications for ecosystems

Atmospheric deposition of particulate organic nitrogen (ONp) is a significant process in the global nitrogen cycle and may be pivotally important for N-limited ecosystems. However, past models largely overlooked the spatial and chemical inhomogeneity of atmospheric ONp and were thus deficient in assessing global ONp impacts. We constructed a comprehensive global model of atmospheric gaseous and particulate organic nitrogen (ON), including the latest knowledge on emissions and secondary formations. Using this model, we simulated global atmospheric ONp abundances consistent with observations. Our estimated global atmospheric ON deposition was 26 Tg N yr-1, predominantly in the form of ONp (23 Tg N yr-1) and mostly from wildfires (37%), oceans (22%) and aqueous productions (17%). Globally, ONp contributed as much as 40% to 80% of the total N deposition downwind of biomass-burning regions. Atmospheric ONp deposition thus constituted the dominant external N supply to the N-limited boreal forests, tundras and the Arctic Ocean, and its importance may be amplified in a future warming climate.

Size distributions of molecular markers for biogenic secondary organic aerosol in urban Beijing

To understand the source, formation, and seasonality of biogenic secondary organic aerosol (BSOA), a nine-stage cascade impactor was utilized to collect size-segregated particulate samples from April 2017 to January 2018 in Beijing, China. BSOA tracers derived from isoprene, monoterpene, and sesquiterpene were measured with gas chromatography-mass spectrometry. Isoprene and monoterpene SOA tracers exhibited significant seasonal variations, with a summer maximum and a winter minimum. Dominance of 2-methyltetrols (isoprene SOA tracers) with a good correlation with levoglucosan (a biomass burning tracer), which was combined with the detection of methyltartaric acids (possible indicators for aged isoprene) in summer, implies possible biomass burning and long-range transport. In contrast, sesquiterpene SOA tracer (β-caryophyllinic acid) was dominant in winter and was probably associated with the local burning of biomass. Bimodal size distributions were observed for most isoprene SOA tracers, consistent with previous laboratory experiments and field studies showing that they can be formed not only in the aerosol phase but also in the gas phase. Monoterpene SOA tracers cis-pinonic acid and pinic acid showed a coarse-mode peak (5.8-9.0 μm) in four seasons due to their volatile nature. Sesquiterpene SOA tracer β-caryophyllinic acid showed a unimodal pattern with a major fine-mode peak (1.1-2.1 μm), which is linked to local biomass burning. The tracer-yield method was used to quantify the contributions of isoprene, monoterpene, and sesquiterpene to secondary organic carbon (SOC) and SOA. The highest isoprene SOC and SOA concentrations occurred in summer (2.00 μgC m^-3 and 4.93 μg m^-3, respectively), contributing to 1.61% of OC and 5.22% of PM_2.5, respectively. These results suggest that BSOA tracers are promising tracers for understanding the source, formation, and seasonality of BSOA.

Environmental factors driving the formation of water-soluble organic aerosols: A comparative study under contrasting atmospheric conditions

Water-soluble organic carbon (WSOC), as major fractions of atmospheric aerosols, have gained attention due to their light-absorption properties. To illustrate the sources and key environmental factors driving WSOC formation under different atmospheric conditions, a comparative study was conducted by summarizing the results obtained from five field campaigns at inland (urban, suburban or regional) sites and a coastal site during different seasons. Organic carbon concentrations varied from 8.5 μg/m³ at the summer regional site to 17.5 μg/m³ at the winter urban site, with 46 %- 89 % of the mass as WSOC. Based on correlation analysis, primary combustion emissions were more important in winter than in summer, and secondary formation was an important source of WSOC during winter, summer and autumn. Atmospheric oxidants (NO₂, O₃), aerosol liquid water (ALW) and ambient RH were important factors influencing the WSOC formation, while their roles varied in different atmospheres. We observed a seasonal transition of atmospheric oxidants dominating the WSOC formation from O₃ and NO₂-driven conditions in summer to NO₂-driven conditions in winter. Elevated ALW or ambient RH generally favor the WSOC formation, while the WSOC dependence of ALW varied among different ALW ranges. As the increasing of ALW or ambient RH, a transition of WSOC formation from "RH/ALW-limited regime" under low-ALW conditions, to "RH/ALW and precursor-driven regime" under medium-ALW/RH, and to "precursor-limited (RH/ALW-excess) regime" were observed for the inland atmospheric conditions. Under the high-RH and ALW conditions in coastal areas, ALW or ambient RH was generally not a limiting factor for WSOC formation.

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

Source characterization of volatile organic compounds in urban Beijing and its links to secondary organic aerosol formation

Volatile organic compounds (VOCs) are precursors for ozone and secondary organic aerosol (SOA) formation, thereby playing a vital role in atmospheric chemistry and urban air quality. To characterize the relationship between VOCs and SOA, organics both in gas and particulate phases were concurrently measured in urban Beijing. The VOCs and organic aerosol (OA) were apportioned into factors with different oxidation levels by applying the factorization analysis on their detailed mass spectra. Six factors of VOCs were identified, including four primary VOCs (PVOC) factors and two secondary VOCs (SVOC) factors. The PVOC factors dominated the total VOCs when the air mass originated in the cleaner northern areas, while SVOC factors dominated for polluted southern air masses. The normalized concentrations of PVOC and primary OA factors showed consistent diurnal variations regardless of air mass directions, owing to the relatively stable local emissions during the experimental period. This contrasted with the secondary factors due to more complex transformation processes. The traffic-related VOCs and solid fuel combustion VOCs negatively correlated with SOA, implying that they may have contributed to the SOA formation through photooxidation. The VOCs in lower oxidation levels were found to have poor correlations with the less oxidized SOA, whereas they correlated strongly to the more oxidized SOA. This implied that the less oxidized SOA may be in a transition state, where its production and loss rates were balanced. These served as products of VOCs oxidation and reactants of more oxidized SOA formation, playing important roles on the VOC to SOA transformation. The identified VOC emission sources and their photochemical production of SOA should be considered in air quality policy planning.

Observation and modeling of organic nitrates on a suburban site in southwest China

Here we report the measurements of two types of organic nitrates (ONs), peroxy nitrates (PNs) and alkyl nitrates (ANs), in Chengdu, China, during summer 2019. The average concentrations of PNs and ANs were 1.3 ± 1.1 ppbv and 0.5 ± 0.3 ppbv during the day, with peaks of 7.7 ppbv and 1.9 ppbv, respectively, which were in the middle and upper end of the reported levels in China. Much higher PNs and ANs concentrations were found during the photochemical pollution period than during the clean period. Box model simulation was capable of reproducing PNs during photochemical pollution episodes but showed overestimation in other periods, which was likely caused by the simplification of PNs sinks. The OH oxidation of aldehydes and ketones was the most important source of the PNs precursors, PAs (peroxyacyl radicals), except for the thermal decomposition of PNs, which was further confirmed by the relative incremental reactivity (RIR) analysis. The model basically reproduced the observed ANs by the refinement of related mechanisms, with isoprene contributing to its formation by 29.2 %. The observed PNs and total oxidants (O_x = NO₂ + O₃) showed a good positive correlation, with a ratio of PNs to O_x of 0.079, indicating a strong suppression of PNs chemistry to ozone formation. The model quantified the suppression of PNs chemistry on the peak ozone production rate by 21.3 % on average and inhibited ozone formation up to 20 ppbv in total. The RIR analysis suggests that the production of both O₃ and ANs was in the VOC-limited regime and highlights the importance of VOC control (especially aromatics) to mitigate photochemical pollution in Chengdu. The study deepens the understanding of photochemical pollution in urban areas of western China and further emphasizes the impacts of ONs chemistry on ozone pollution.

Photolysis of Gas-Phase Atmospherically Relevant Monoterpene-Derived Organic Nitrates

Organic nitrates (ONs) can impact spatial distribution of reactive nitrogen species and ozone formation in the atmosphere. While photolysis of ONs is known to result in the release of NO₂ back to the atmosphere, the photolysis rate constants and mechanisms of monoterpene-derived ONs (MT-ONs) have not been well constrained. We investigated the gas-phase photolysis of three synthetic ONs derived from α-pinene, β-pinene, and d-limonene through chamber experiments. The measured photolysis rate constants ranged from (0.55 ± 0.10) × 10^-5 to (2.3 ± 0.80) × 10^-5 s^-1 under chamber black lights. When extrapolated to solar spectral photon flux at a solar zenith angle of 28.14° in summer, the photolysis rate constants were in the range of (4.1 ± 1.4) × 10^-5 to (14 ± 6.7) × 10^-5 s^-1 (corresponding to lifetimes of 2.0 ± 0.96 to 6.8 ± 2.4 h) and (1.7 ± 0.60) × 10^-5 to (8.3 ± 4.0) ×10^-5 s^-1 (3.3 ± 1.6 to 17 ± 6.0 h lifetimes) by using wavelength-dependent and average quantum yields, respectively. Photolysis mechanisms were proposed based on major products detected during photolysis. A zero-dimensional box model was further employed to simulate the photolysis of α-pinene-derived ON under ambient conditions. We found that more than 99% of α-pinene-derived ON can be converted to inorganic nitrogen within 12 h of irradiation and ozone was formed correspondingly. Together, these findings show that photolysis is an important atmospheric sink for MT-ONs and highlight their role in NO_x recycling and ozone chemistry.

Distinct diurnal chemical compositions and formation processes of individual organic-containing particles in Beijing winter

Organic aerosols (OA) are major components of fine particulate matter, yet their formation mechanism remains unclear, especially in polluted environments. In this study, we investigated the diurnal chemical compositions and formation processes of OA in carbonaceous particles during winter in Beijing using aerosol time-of-flight mass spectrometry. We found that 84.5% of the measured carbonaceous particles underwent aging processes, characterized by larger diameter and more secondary species compared to fresh carbonaceous particles, and presented different chemical compositions of OA in the daytime and nighttime. During the day, under high O₃ concentrations, organosulfates and oligomers existed in the aged carbonaceous particles, which were mixed with a higher signal of nitrate compared with sulfate. At night, under high relative humidity, distinct spectral signatures of hydroxymethanesulfonate and organic nitrogen compounds, and minor signals of other hydroxyalkylsulfonates and high molecular weight organic compounds were present in the aged carbonaceous particles, which were mixed with a higher signal of sulfate compared with nitrate. Our results indicated that photochemistry contributed to OA formation in the daytime, while aqueous chemistry played an important role in OA formation in the nighttime. The findings can help improve the performance of air quality and climate models on OA simulation.

Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer

Particulate organic nitrate (pON) can be a major part of secondary organic aerosol (SOA) and is commonly quantified by indirect means from aerosol mass spectrometer (AMS) data. However, pON quantification remains challenging. Here, we set out to quantify and characterize pON in the boreal forest, through direct field observations at Station for Measuring Ecosystem Atmosphere Relationships (SMEAR) II in Hyytiälä, Finland, and targeted single-precursor laboratory studies. We utilized a long time-of-flight AMS (LToF-AMS) for aerosol chemical characterization, with a particular focus to identify C _x H _y O _z N⁺ ("CHON⁺") fragments. We estimate that during springtime at SMEAR II, pON (including both the organic and nitrate part) accounts for ∼10% of the particle mass concentration (calculated by the NO⁺/NO₂ ⁺ method) and originates mainly from the NO₃ radical oxidation of biogenic volatile organic compounds. The majority of the background nitrate aerosol measured is organic. The CHON⁺ fragment analysis was largely unsuccessful at SMEAR II, mainly due to low concentrations of the few detected fragments. However, our findings may be useful at other sites as we identified 80 unique CHON⁺ fragments from the laboratory measurements of SOA formed from NO₃ radical oxidation of three pON precursors (β-pinene, limonene, and guaiacol). Finally, we noted a significant effect on ion identification during the LToF-AMS high-resolution data processing, resulting in too many ions being fit, depending on whether tungsten ions (W⁺) were used in the peak width determination. Although this phenomenon may be instrument-specific, we encourage all (LTOF-) AMS users to investigate this effect on their instrument to reduce the possibility of incorrect identifications.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH₂ OO, CH₃ CHOO, (CH₃ )₂ COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH₂ OO and (CH₃ )₂ COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing ¹ ππ* state, and how this varies with CI, with conformer and with excitation wavelength. The later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O-O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

Oxidation of Catechols at the Air-Water Interface by Nitrate Radicals

Abundant substituted catechols are emitted to, and created in, the atmosphere during wildfires and anthropogenic combustion and agro-industrial processes. While ozone (O₃) and hydroxyl radicals (HO^•) efficiently react in a 1 μs contact time with catechols at the air-water interface, the nighttime reactivity dominated by nitrate radicals (NO₃) remains unexplored. Herein, online electrospray ionization mass spectrometry (OESI-MS) is used to explore the reaction of NO₃(g) with a series of representative catechols (catechol, pyrogallol, 3-methylcatechol, 4-methylcatechol, and 3-methoxycatechol) on the surface of aqueous microdroplets. The work detects the ultrafast generation of nitrocatechol (aromatic) compounds, which are major constituents of atmospheric brown carbon. Two mechanisms are proposed to produce nitrocatechols, one (equivalent to H atom abstraction) following fast electron transfer from the catechols (QH₂) to NO₃, forming NO₃^- and QH₂^•+ that quickly deprotonates into a semiquinone radical (QH^•). The second mechanism proceeds via cyclohexadienyl radical intermediates from NO₃ attack to the ring. Experiments in the pH range from 4 to 8 showed that the production of nitrocatechols was favored under the most acidic conditions. Mechanistically, the results explain the interfacial production of chromophoric nitrocatechols that modify the absorption properties of tropospheric particles, making them more susceptible to photooxidation, and alter the Earth's radiative forcing.

Peroxy Radical Autoxidation and Sequential Oxidation in Organic Nitrate Formation during Limonene Nighttime Oxidation

Limonene is an abundant monoterpene released into the atmosphere via biogenic emissions and biomass burning. However, the atmospheric oxidation and secondary organic aerosol (SOA) formation mechanisms of limonene, especially during nighttime, remain largely understudied. In this work, limonene was oxidized synergistically by ozone (O₃) and nitrate radicals (NO₃) in a flow tube reactor and a continuous flow stirred tank reactor. Upon oxidation, many highly oxidized organic nitrates and nitrooxy peroxy radicals (RO₂) were observed in the gas phase within 1 min. Combining quantum chemical calculations with kinetic simulations, we found that the primary nitrooxy RO₂ (C₁₀H₁₆NO₅) through NO₃ addition at the more substituted endocyclic double bond and at the exocyclic double bond (previously considered as minor pathways) can undergo autoxidation with rate constants of around 0.02 and 20 s^-1 at 298 K, respectively. These pathways could explain a major portion of the observed highly oxidized organic nitrates. In the SOA, highly oxidized mono- and dinitrates (e.g., C₁₀H₁₇NO_7-8 and C₁₀H₁₆,₁₈N₂O_8-10) make up a significant contribution, highlighting nitrooxy RO₂ autoxidation and sequential NO₃ oxidation of limonene. The same organic nitrates are also observed in ambient aerosol during biomass burning and nighttime in the southeastern United States. Therefore, the present work provides new insights into the nighttime oxidation of limonene and SOA formation in the atmosphere.

Chemistry of Secondary Organic Aerosol Formation from Reactions of Monoterpenes with OH Radicals in the Presence of NOx

The oxidation of volatile organic compounds (VOCs), which are emitted to the atmosphere from natural and anthropogenic sources, leads to the formation of ozone and secondary organic aerosol (SOA) particles that impact air quality and climate. In the study reported here, we investigated the products of the reactions of five biogenic monoterpenes with OH radicals (an important daytime oxidant) under conditions that mimic the chemistry that occurs in polluted air, and developed mechanisms to explain their formation. Experiments were conducted in an environmental chamber, and information on the identity of gas-phase molecular products was obtained using online mass spectrometry, while liquid chromatography and two methods of functional group analysis were used to characterize the SOA composition. The gas-phase products of the reactions were similar to those formed in our previous studies of the reactions of these monoterpenes with NO₃ radicals (an important nighttime oxidant), in that they all contained various combinations of nitrate, carbonyl, hydroxyl, ester, and ether groups. But in spite of this, less SOA was formed in OH/NO_x reactions and it was composed of monomers, while SOA formed in NO₃ radical reactions consisted of acetal and hemiacetal oligomers formed by particle-phase accretion reactions. In addition, it appeared that some monomers underwent particle-phase hydrolysis, whereas oligomers did not. These differences are due primarily to the arrangement of hydroxyl, carbonyl, nitrate, and ether groups in the monomers, which can in turn be explained by differences in OH and NO₃ radical reaction mechanisms. The results provide insight into the impact of VOC structure on the amount and composition of SOA formed by atmospheric oxidation, which influence important aerosol properties such as volatility and hygroscopicity.

Secondary Organic Aerosol Mass Yields from NO3 Oxidation of α-Pinene and Δ-Carene: Effect of RO2 Radical Fate

Dark chamber experiments were conducted to study the SOA formed from the oxidation of α-pinene and Δ-carene under different peroxy radical (RO₂) fate regimes: RO₂ + NO₃, RO₂ + RO₂, and RO₂ + HO₂. SOA mass yields from α-pinene oxidation were <1 to ∼25% and strongly dependent on available OA mass up to ∼100 μg m^-3. The strong yield dependence of α-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Δ-carene + NO₃ were consistently higher, ranging from ∼10-50% with some dependence on OA for <25 μg m^-3. Explicit kinetic modeling including vapor wall losses was conducted to enable comparisons across VOC precursors and RO₂ fate regimes and to determine atmospherically relevant yields. Furthermore, SOA yields were similar for each monoterpene across the nominal RO₂ + NO₃, RO₂ + RO₂, or RO₂ + HO₂ regimes; thus, the volatility basis sets (VBS) constructed were independent of the chemical regime. Elemental O/C ratios of ∼0.4-0.6 and nitrate/organic mass ratios of ∼0.15 were observed in the particle phase for both monoterpenes in all regimes, using aerosol mass spectrometer (AMS) measurements. An empirical relationship for estimating particle density using AMS-derived elemental ratios, previously reported in the literature for non-nitrate containing OA, was successfully adapted to organic nitrate-rich SOA. Observations from an NO₃^- chemical ionization mass spectrometer (NO₃-CIMS) suggest that Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semivolatile species, when reacted with NO₃, regardless of RO₂ regime. The similar Δ-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multistep processes such as alkoxy radical isomerization and decomposition may play a role in the formation of SOA from Δ-carene + NO₃. The scarcity of peroxide functional groups (on average, 14% of C₁₀ groups carried a peroxide functional group in one test experiment in the RO₂ + RO₂ regime) appears to rule out a major role for autoxidation and organic peroxide (ROOH, ROOR) formation. The consistently substantially lower SOA yields observed for α-pinene + NO₃ suggest such pathways are less available for this precursor. The marked and robust regime-independent difference in SOA yield from two different precursor monoterpenes suggests that in order to accurately model SOA production in forested regions the chemical mechanism must feature some distinction among different monoterpenes.

Particulate organic nitrates at Mount Tai in winter and spring: Variation characteristics and effects of mountain-valley breezes and elevated emission sources

Particulate organic nitrates, among the major components of secondary organic aerosols and fine particles, play important roles in regional nitrogen cycle, ozone budget, and cloud condensation nuclei formation. However, the pollution characteristics of particulate organic nitrates at mountain areas and the effects of anthropogenic pollutant transport remain poorly understood. In this study, field sampling and measurements were conducted at a high-elevation mountain site over North China Plain in winter and spring. Total five kinds of particulate organic nitrates in fine particles were determined by ultra-high performance liquid chromatography-electrospray mass spectrometry. The average total concentrations of particulate organic nitrates were 330 ± 121 ng m^-3 and 247 ± 63 ng m^-3 in winter and spring. The monoterpene-derived organic nitrates were the dominant components in both seasons with their contribution higher than 70%, accounting for 1.2 ± 0.8% and 2.0 ± 1.0% in organic aerosols in winter and spring, respectively. The significantly higher levels of particulate organic nitrates in winter than spring was ascribed to the strong effects of mountain-valley breezes and coal combustion plumes. The increasing concentrations of NO_x and particulate matters brought by the valley breeze at daytime facilitated the formation of MHN215, OAKN359, and OAHN361, while the rising SO₂ abundance and the sulfate aerosols transported by elevated emission sources affected the formation of MDCN247 at nighttime.

Anthropogenic monoterpenes aggravating ozone pollution

Monoterpenes have been known to have a critical influence on air quality and climate change through their impact on the formation of fine particles. Here we present field evidence that monoterpene oxidations largely enhanced local ozone production in a regional site in eastern China. The observed monoterpene was most likely from biomass burning rather than biogenic emissions, as indicated by the high correlation with CO at night-time, and the observed ratio of these two species was consistent with previously determined values from biomass burning experiments. Fast monoterpene oxidations were determined experimentally based on direct radical measurements, leading to a daily ozone enhancement of 4–18 parts per billion by volume (ppb), which was 6%–16% of the total ozone production, depending on the speciation of monoterpenes. It demonstrates that the previously overlooked anthropogenic monoterpenes make an important contribution to O₃ production in eastern China. The role could possibly be important at similar locations across China and other parts of the world that are characterized by massive emissions, especially where there are high NO_x levels. Our results highlight that anthropogenic monoterpenes should be taken into account when proceeding with the coordinated mitigation of O₃ and particulate matter pollution.

A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates

Secondary organic aerosols (SOA) are crucial constitution of fine particulate matter (PM), which are mainly derived from photochemical oxidation products of primary organic matter and volatile organic compounds (VOCs), and can induce terrible impacts to human health, air quality and climate change. As we know, organosulfates (OSs) and organic nitrates (ON) are important contributors for SOA formation, which could be possibly produced through various pathways, resulting in extremely complex formation mechanism of SOA. Although plenty of research has been focused on the origins, spatial distribution and formation mechanisms of SOA, a comprehensive and systematic understanding of SOA formation in the atmosphere remains to be detailed explored, especially the most important OSs and ON dedications. Thus, in this review, we systematically summarize the recent research about origins and formation mechanisms of OSs and ON, and especially focus on their contribution to SOA, so as to have a clearer understanding of the origin, spatial distribution and formation principle of SOA. Importantly, we interpret the complex interaction with coexistence effect of SO_x and NO_x on SOA formation, and emphasize the future insights for SOA research to expect a more comprehensive theory and practice to alleviate SOA burden.

Probing of sequential atmospheric degradation of chlorine radical initiated 1,8-cineole in the presence of O2 and NO radical with the emission of secondary pollution

1,8-cineole is an essential monoterpene cyclic ether which is released into the troposphere by many types of plants. It interacts with several atmospheric oxidants because of which is removed from the troposphere via oxidation. The oxidation of 1,8-cineole with Cl radical and the subsequent addition of atmospheric O₂ and NO radical with the intermediates are studied using the quantum chemical method. Further, the thermodynamic parameters of 1,8-cineole, such as enthalpy and Gibbs free energy are calculated for all initial and subsequent reactions to facilitate perspicacity. The dissociation and formation of chemical bonds during H abstraction from 1,8-cineole at C2, C6, and C8 sites are described using Mayer bond order analysis. The reaction force analysis demonstrates that the structural rearrangement is dominant with the yield percentages of 85%, 50.80%, and 96.9% over electron reordering with the yield percentages of 15%, 49.19%, and 3.03% respectively in the H abstraction reaction of 1,8-cineole. In the temperature range of 278-350 K, the total CVT/SCT rate constant is calculated to be 2.94 × 10^-12 cm³/molecule/sec, which is consistent with the experimentally available value of 2.2 × 10^-10 cm³/molecule/sec. At 298 K, branching ratios of rate constant of alkyl radical intermediates I1A, I1B, and I1C are calculated with the percentage of 42.19%, 21.52%, and 36.29% respectively, which suggest that the Cl addition to the C2 site contributes more to the total rate constant rather than the other two sites (C6 and C8). The lifetime of 1,8-cineole is calculated to be 5.2 weeks, implies that the 1,8-cineole may be readily destroyed in the atmosphere after it is released. Secondary pollutants formed from this degradation mechanism may be harmful to the environment and the living things.

Seasonality of Aerosol Sources Calls for Distinct Air Quality Mitigation Strategies

An Aerosol Chemical Speciation Monitor (ACSM) was deployed to investigate the temporal variability of non-refractory particulate matter (NR-PM₁) in the coastal city of Galway, Ireland, from February to July 2016. Source apportionment of the organic aerosol (OA) was performed using the newly developed rolling PMF strategy and was compared with the conventional seasonal PMF. Primary OA (POA) factors apportioned by rolling and seasonal PMF were similar. POA factors of hydrocarbon-like OA (HOA), peat, wood, and coal were associated with domestic heating, and with an increased contribution to the OA mass in winter. Even in summer, sporadic heating events occurred with similar diurnal patterns to that in winter. Two oxygenated OA (OOA) factors were resolved, including more-oxygenated OOA and less-oxygenated OOA (i.e., MO-OOA and LO-OOA, accordingly) which were found to be the dominant OA factors during summer. On average, MO-OOA accounted for 62% of OA and was associated with long-range transport in summer. In summer, compared to rolling PMF, the conventional seasonal PMF over-estimated LO-OOA by nearly 100% while it underestimated MO-OOA by 30%. The results from this study show residential heating and long-range transport alternately dominate the submicron aerosol concentrations in this coastal city, requiring different mitigation strategies in different seasons.

Development and Assessment of a High-Resolution Biogenic Emission Inventory from Urban Green Spaces in China

Biogenic volatile organic compound (BVOC) emissions have long been known to play vital roles in modulating the formation of ozone and secondary organic aerosols (SOAs). While early studies have evaluated their impact globally or regionally, the BVOC emissions emitted from urban green spaces (denoted as U-BVOC emissions) have been largely ignored primarily due to the failure of low-resolution land cover in resolving such processes, but also because their important contribution to urban BVOCs was previously unrecognized. In this study, by utilizing a recently released high-resolution land cover dataset, we develop the first set of emission inventories of U-BVOCs in China at spatial resolutions as high as 1 km. This new dataset resolved densely distributed U-BVOCs in urban core areas. The U-BVOC emissions in megacities could account for a large fraction of total BVOC emissions, and the good agreement of the interannual variations between the U-BVOC emissions and ozone concentrations over certain regions stresses their potentially crucial role in influencing ozone variations. The newly constructed U-BVOC emission inventory is expected to provide an improved dataset to enable the research community to re-examine the modulation of BVOCs on the formation of ozone, SOA, and atmospheric chemistry in urban environments.

Secondary aerosol formation during the dark oxidation of residential biomass burning emissions

Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established. We performed atmospheric simulation chamber experiments on the chemical oxidation of residential biomass burning emissions under dark conditions. Biomass burning organic aerosol was found to age under dark conditions, with its oxygen-to-carbon ratio increasing by 7–34% and producing 1–38 μg m⁻³ of secondary organic aerosol (5–80% increase over the fresh organic aerosol) after 30 min of exposure to NO₃ radicals in the chamber (corresponding to 1–3 h of exposure to typical nighttime NO₃ radical concentrations in an urban environment). The average mass concentration of SOA formed under dark-oxidation conditions was comparable to the mass concentration formed after 3 h (equivalent to 7–10 h of ambient exposure) under ultraviolet lights (6 μg m⁻³ or a 47% increase over the emitted organic aerosol concentration). The dark-aging experiments showed a substantial increase in secondary nitrate aerosol (0.12–3.8 μg m⁻³), 46–100% of which is in the form of organic nitrates. The biomass burning aerosol pH remained practically constant at 2.8 throughout the experiment. This value promotes inorganic nitrate partitioning to the particulate phase, potentially contributing to the buildup of nitrate aerosol in the boundary layer and enhancing long-range transport. These results suggest that oxidation through reactions with the NO₃ radical is an additional secondary aerosol formation pathway in biomass burning emission plumes that should be accounted for in atmospheric chemical-transport models.

Biomass burning emissions age rapidly in the dark due to oxidation reactions with nitrate radicals.

Computational Investigation of the Formation of Peroxide (ROOR) Accretion Products in the OH- and NO3-Initiated Oxidation of α-Pinene

The formation of accretion products (“dimers”) from recombination reactions of peroxyl radicals (RO₂) is a key step in the gas-phase generation of low-volatility vapors, leading to atmospheric aerosol particles. We have recently demonstrated that this recombination reaction very likely proceeds via an intermediate complex of two alkoxy radicals (RO···OR′) and that the accretion product pathway involves an intersystem crossing (ISC) of this complex from the triplet to the singlet surface. However, ISC rates have hitherto not been computed for large and chemically complex RO···OR′ systems actually relevant to atmospheric aerosol formation. Here, we carry out systematic conformational sampling and ISC rate calculations on ³(RO···OR′) clusters formed in the recombination reactions of different diastereomers of the first-generation peroxyl radicals originating in both OH- and NO₃-initiated reactions of α-pinene, a key biogenic hydrocarbon for atmospheric aerosol formation. While we find large differences between the ISC rates of different diastereomer pairs, all systems have ISC rates of at least 10⁶ s^–1, and many have rates exceeding 10¹⁰ s^–1. Especially the latter value demonstrates that accretion product formation via the suggested pathway is a competitive process also for α-pinene-derived RO₂ and likely explains the experimentally observed gas-phase formation of C₂₀ compounds in α-pinene oxidation.

Contribution of Organic Nitrates to Organic Aerosol over South Korea during KORUS-AQ

The role of anthropogenic NOx emissions in secondary organic aerosol (SOA) production is not fully understood but is important for understanding the contribution of emissions to air quality. Here, we examine the role of organic nitrates (RONO2) in SOA formation over the Korean Peninsula during the Korea-United States Air Quality field study in Spring 2016 as a model for RONO2 aerosol in cities worldwide. We use aircraft-based measurements of the particle phase and total (gas + particle) RONO2 to explore RONO2 phase partitioning. These measurements show that, on average, one-fourth of RONO2 are in the condensed phase, and we estimate that ≈15% of the organic aerosol (OA) mass can be attributed to RONO2. Furthermore, we observe that the fraction of RONO2 in the condensed phase increases with OA concentration, evidencing that equilibrium absorptive partitioning controls the RONO2 phase distribution. Lastly, we model RONO2 chemistry and phase partitioning in the Community Multiscale Air Quality modeling system. We find that known chemistry can account for one-third of the observed RONO2, but there is a large missing source of semivolatile, anthropogenically derived RONO2. We propose that this missing source may result from the oxidation of semi- and intermediate-volatility organic compounds and/or from anthropogenic molecules that undergo autoxidation or multiple generations of OH-initiated oxidation.

Herbicide Drift from Genetically Engineered Herbicide-Tolerant Crops

In recent years, off-target herbicide drift has been increasingly reported to lead to damage to nontarget vegetation in the U.S. These reports have coincided with the widespread adoption of genetically modified crops with new herbicide-tolerance traits. Planting crops with these traits may indirectly lead to increased drift both by increasing the use of the corresponding herbicides and by facilitating their use as postemergence herbicides later in the season. While extensive efforts have aimed to reduce herbicide drift, critical uncertainties remain regarding the physiochemical phenomena that drive the entry of herbicides into the atmosphere as well as the atmospheric processes that may influence short- and long-range transport. Resolving these uncertainties will support the development of effective approaches to reduce herbicide drift.

Highly Oxygenated Organic Nitrates Formed from NO3 Radical-Initiated Oxidation of β-Pinene

The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO₃) are major night-time sources of organic nitrates and secondary organic aerosols (SOA) in regions influenced by BVOC and anthropogenic emissions. In this study, the formation of gas-phase highly oxygenated organic molecules-organic nitrates (HOM-ON) from NO₃-initiated oxidation of a representative monoterpene, β-pinene, was investigated in the SAPHIR chamber (Simulation of Atmosphere PHotochemistry In a large Reaction chamber). Six monomer (C = 7-10, N = 1-2, O = 6-16) and five accretion product (C = 17-20, N = 2-4, O = 9-22) families were identified and further classified into first- or second-generation products based on their temporal behavior. The time lag observed in the peak concentrations between peroxy radicals containing odd and even number of oxygen atoms, as well as between radicals and their corresponding termination products, provided constraints on the HOM-ON formation mechanism. The HOM-ON formation can be explained by unimolecular or bimolecular reactions of peroxy radicals. A dominant portion of carbonylnitrates in HOM-ON was detected, highlighting the significance of unimolecular termination reactions by intramolecular H-shift for the formation of HOM-ON. A mean molar yield of HOM-ON was estimated to be 4.8% (-2.6%/+5.6%), suggesting significant HOM-ON contributions to the SOA formation.

Gas- and Particle-Phase Products and Their Mechanisms of Formation from the Reaction of Δ-3-Carene with NO3 Radicals

Monoterpenes are a major component of the large quantities of biogenic volatile organic compounds that are emitted to the atmosphere each year. They have a variety of structures, which influences their subsequent reactions with OH radicals, O₃, or NO₃ radicals and the tendency for these reactions to form secondary organic aerosol (SOA). Here we report the results of an environmental chamber study of the reaction of Δ-3-carene, an abundant unsaturated C₁₀ bicyclic monoterpene, with NO₃ radicals, a major nighttime oxidant. Gas- and particle-phase reaction products were analyzed in real time and offline by using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. The results were used to identify and quantify functional groups and molecular products and to develop gas- and particle-phase reaction mechanisms to explain their formation. Identified gas-phase products were all first-generation ring-retaining and ring-opened compounds (ten C₁₀ and one C₉ monomers) with 2-4 functional groups and one C₂₀ dinitrooxydialkyl peroxide dimer. Upon partitioning to the particle phase, the monomers reacted further to form oligomers consisting almost entirely of C₂₀ acetal and hemiacetal dimers, with those formed from a hydroxynitrate and hydroxycarbonyl nitrate comprising more than 50% of the SOA mass. The SOA contained an average of 0.94, 0.71, 0.15, 0.11, 0.16, 0.13, and 7.80 nitrate, carbonyl, hydroxyl, carboxyl, ester, peroxide, and methylene groups per C₁₀ monomer and was formed with a mass yield of 56%. These results have important similarities and differences to those obtained from a previous similar study of the reaction of β-pinene and yield new insights into the effects of monoterpene structure on gas- and particle-phase reactions that can lead to the formation of a large variety of multifunctional products and significant amounts of SOA.

Characterizing nitrate radical budget trends in Beijing during 2013-2019

Nitrate (NO₃) radical is an important oxidant in the atmosphere as it regulates the NO_x budget and impacts secondary pollutant formation. Here, a long-term observational dataset of NO₃-related species at an urban site in Beijing was used to investigate changes in the NO₃ budget and their atmospheric impacts during 2013-2019, in this period the Clean Air Actions Plan was carried out in China. We found that (1) changes in NO₃ precursors (NO₂ and O₃) led to a significant increase in NO₃ formation in the surface layer in winter but a decrease in summer; (2) a reduction in NO_x promoted thermal equilibrium, favoring the formation of NO₃ rather than dinitrogen pentoxide (N₂O₅). The simultaneous decrease in PM_2.5, during these years, further weakened the N₂O₅ heterogeneous uptake; (3) a box model simulation revealed that both the reactions of NO₃ with volatile organic compounds (VOC) and N₂O₅ uptake were weakened in summer, implying that the policy actions implemented help to moderate secondary aerosol formation caused by NO₃ and N₂O₅ chemistry in summer; and (4) during winter, both NO₃ + VOC and N₂O₅ uptake were enhanced. Specifically, for the N₂O₅ uptake, the rapid increase in NO₃ production, or to some extent, NO₃ oxidation capacity, far outweighed the negative shift effect, leading to a net enhancement of N₂O₅ uptake in winter, which indicates that the action policy implemented led to an adverse effect on particulate nitrate formation via N₂O₅ uptake in winter. This may explain the persistent winter particulate nitrate pollution in recent years. Our results highlight the systematic changes in the NO₃ budget between 2013 and 2019 in Beijing, which subsequently affect secondary aerosol formation in different seasons.

Abundance of NO3 Derived Organo-Nitrates and Their Importance in the Atmosphere

The chemistry of the nitrate radical and its contribution to organo-nitrate formation in the troposphere has been investigated using a mesoscale 3-D chemistry and transport model, WRF-Chem-CRI. The model-measurement comparisons of NO2, ozone and night-time N2O5 mixing ratios show good agreement supporting the model’s ability to represent nitrate (NO3) chemistry reasonably. Thirty-nine organo-nitrates in the model are formed exclusively either from the reaction of RO2 with NO or by the reaction of NO3 with alkenes. Temporal analysis highlighted a significant contribution of NO3-derived organo-nitrates, even during daylight hours. Night-time NO3-derived organo-nitrates were found to be 3-fold higher than that in the daytime. The reactivity of daytime NO3 could be more competitive than previously thought, with losses due to reaction with VOCs (and subsequent organo-nitrate formation) likely to be just as important as photolysis. This has highlighted the significance of NO3 in daytime organo-nitrate formation, with potential implications for air quality, climate and human health. Estimated atmospheric lifetimes of organo-nitrates showed that the organo-nitrates act as NOx reservoirs, with particularly short-lived species impacting on air quality as contributors to downwind ozone formation.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Formation of Oxidized Gases and Secondary Organic Aerosol from a Commercial Oxidant-Generating Electronic Air Cleaner

The COVID-19 pandemic increased the demand for indoor air cleaners. While some commercial electronic air cleaners can be effective in reducing primary pollutants and inactivating bioaerosol, studies on the formation of secondary products from oxidation chemistry during their use are limited. Here, we measured oxygenated volatile organic compounds (OVOCs) and the chemical composition of particles generated from a hydroxyl radical generator in an office. During operation, enhancements in OVOCs, especially low-molecular-weight organic acids, were detected. Rapid increases in particle number and mass concentrations were observed, corresponding to the formation of highly oxidized secondary organic aerosol (SOA) (O:C ∼ 1.3), with an enhanced signal at m/z 44 (CO2+) in the organic mass spectra. These results suggest that organic acids generated during VOC oxidation contributed to particle nucleation and SOA formation. Nitrate, sulfate, and chloride also increased during the oxidation without a corresponding increase in ammonium, suggesting organic nitrate, organic sulfate, and organic chloride formation. As secondary species are reported to have detrimental health effects, further studies should not be limited to the inactivation of bioaerosol or reduction of particular VOCs, but should also evaluate potential OVOCs and SOA formation from electronic air cleaners in different indoor environments.

Comprehensive evaluations of diurnal NO2 measurements during DISCOVER-AQ 2011: effects of resolution-dependent representation of NO x emissions

Nitrogen oxides (NO _x =NO+NO₂) play a crucial role in the formation of ozone and secondary inorganic and organic aerosols, thus affecting human health, global radiation budget, and climate. The diurnal and spatial variations in NO₂ are functions of emissions, advection, deposition, vertical mixing, and chemistry. Their observations, therefore, provide useful constraints in our understanding of these factors. We employ a Regional chEmical and trAnsport model (REAM) to analyze the observed temporal (diurnal cycles) and spatial distributions of NO₂ concentrations and tropospheric vertical column densities (TVCDs) using aircraft in situ measurements and surface EPA Air Quality System (AQS) observations as well as the measurements of TVCDs by satellite instruments (OMI: the Ozone Monitoring Instrument; GOME-2A: Global Ozone Monitoring Experiment - 2A), ground-based Pandora, and the Airborne Compact Atmospheric Mapper (ACAM) instrument in July 2011 during the DISCOVER-AQ campaign over the Baltimore-Washington region. The model simulations at 36 and 4 km resolutions are in reasonably good agreement with the regional mean temporospatial NO₂ observations in the daytime. However, we find significant overestimations (underestimations) of model-simulated NO₂ (O₃) surface concentrations during night-time, which can be mitigated by enhancing nocturnal vertical mixing in the model. Another discrepancy is that Pandora-measured NO₂ TVCDs show much less variation in the late afternoon than simulated in the model. The higher-resolution 4 km simulations tend to show larger biases compared to the observations due largely to the larger spatial variations in NO _x emissions in the model when the model spatial resolution is increased from 36 to 4 km. OMI, GOME-2A, and the high-resolution aircraft ACAM observations show a more dispersed distribution of NO₂ vertical column densities (VCDs) and lower VCDs in urban regions than corresponding 36 and 4 km model simulations, likely reflecting the spatial distribution bias of NO _x emissions in the National Emissions Inventory (NEI) 2011.

Chemical Composition of Gas and Particle Phase Products of Toluene Photooxidation Reaction under High OH Exposure Condition

In the current study, the photooxidation reaction of toluene (C7H8) was investigated in a Potential Aerosol Mass Oxidation Flow Reactor (PAM OFR). The hydroxyl radical (OH) exposure of toluene in the PAM OFR ranged from 0.4 to 1.4 × 1012 molec cm−3 s, which is equivalent to 3 to 12 days of atmospheric oxidation. A proton transfer reaction-mass spectrometer (PTR-MS) and a scanning mobility particle sizer (SMPS) were used to study the gas-phase products formed and particle number changes of the oxidation reaction in PAM OFR. The secondary organic aerosol (SOA) formed in the PAM OFR was also collected for off-line chemical analysis. Key gas-phase reaction products of toluene, including glyoxal, methyl glyoxal, unsaturated carbonyl compounds, and benzaldehyde, were identified by the PTR-MS. Second generation products, including acetic acid, formaldehyde, formic acid, and acetaldehyde, were also detected. By comparing the mass spectrums obtained under different OH exposures and relative humidity (RH), changes in the two parameters have minimal effects on the composition of gas-phase products formed, expect for the spectrum obtained at OH exposure of 0.4 × 1012 cm−3 s and RH = 17%, which is slightly different from other spectrums. SMPS results showed that particle mass concentration increases with increasing OH exposure, while particle number concentration first increases and then decreases with increasing OH exposure. This result probably suggests the formation of oligomers at high OH exposure conditions. Off-line chemical analysis of the SOA sample was dominated by C4 diacids, including malic acid, citramalic acid, and tartaric acid. The well-known toluene SOA marker 2,3-Dihydroxy-4-oxopentanoic acid, as well as 2,3-dihydroxyglutaric acid, which has not been identified in previous toluene photooxidation experiments, were also detected in the SOA sample. Our results showed good agreements with the results of previous smog chamber studies of toluene photooxidation reaction, and they suggested that using PAM OFR for studies of oxidation reaction of different VOCs can be atmospherically relevant.

Chemical Synthesis of Multifunctional Air Pollutants: Terpene-Derived Nitrooxy Organosulfates

Nitrooxy organosulfates derived from terpenes (NOS_TP) represent an important class of products formed between anthropogenic pollution (e.g., SO₂ and NO_x) and natural emissions. NOS_TP compounds have been consistently detected in atmospheric environments under varying urban influences. Their chemical linkages to both anthroposphere and biosphere make them valuable markers for tracking anthroposphere-biosphere interactions. However, their quantification, formation, and transformation kinetics in atmospheric aerosols are hindered due to the lack of NOS_TP standards. In this work, we developed two routes for the first concise chemical synthesis of eight NOS_TP from terpenes including α-pinene, β-pinene, limonene, limonaketone, and β-caryophyllene. Subsequently, six of the synthesized NOS_TP were for the first time positively identified in ambient aerosol samples, clarifying certain misidentifications in previous studies. More significantly, the availability of authentic standards allows irrefutable observation of three carbon skeleton-rearranged NOS_TP, two derived from α-pinene, and one derived from β-caryophyllene, revealing the occurrence of previously unrecognized transformation pathways in the formation of NOS_TP. Two synthesized NOS_TP from β-pinene and limonene could not be detected, likely due to rapid hydrolysis of their immediate hydroxynitrate precursors outcompeting sulfation. Such mechanistic evidence is valuable in understanding the atmospheric chemistry of NOS_TP and related compounds. This work demonstrates the usefulness of authentic standards in probing the NOS_TP formation mechanisms in the atmosphere. Comparison of NOS_TP ambient samples collected from four Chinese cities in two winter months indicates that anthropogenic chemical factors could outcompete terpene emissions in the formation of NOS_TP.

Anthropogenic-Biogenic Interactions at Night: Enhanced Formation of Secondary Aerosols and Particulate Nitrogen- and Sulfur-Containing Organics from β-Pinene Oxidation

Mixing of anthropogenic gaseous pollutants and biogenic volatile organic compounds impacts the formation of secondary aerosols, but still in an unclear manner. The present study explores secondary aerosol formation via the interactions between β-pinene, O₃, NO₂, SO₂, and NH₃ under dark conditions. Results showed that aerosol yield can be largely enhanced by more than 330% by NO₂ or SO₂ but slightly enhanced by NH₃ by 39% when the ratio of inorganic gases to β-pinene ranged from 0 to 1.3. Joint effects of NO₂ and SO₂ and SO₂ and NH₃ existed as aerosol yields increased with NO₂ but decreased with NH₃ when SO₂ was kept constant. Infrared spectra showed nitrogen-containing aerosol components derived from NO₂ and NH₃ and sulfur-containing species derived from SO₂. Several particulate organic nitrates (MW 215, 229, 231, 245), organosulfates (MW 250, 264, 280, 282, 284), and nitrooxy organosulfates (MW 295, 311, 325, 327, and 343) were identified using high-resolution orbitrap mass spectrometry in NO₂ and SO₂ experiments, and their formation mechanism is discussed. Most of these nitrogen- and sulfur-containing species have been reported in ambient particles. Our results suggest that the complex interactions among β-pinene, O₃, NO₂, SO₂, and NH₃ during the night might serve as a potential pathway for the formation of particulate nitrogen- and sulfur-containing organics, especially in polluted regions with both anthropogenic and biogenic influences.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O₃, and Cl atom, HONO, NO₃, N₂O₅, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO₃, N₂O₅, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

Exploring the inorganic and organic nitrate aerosol formation regimes at a suburban site on the North China Plain

Nitrate-driven aerosol pollution frequently occurs during winter over the North China Plain (NCP). Extensive studies have focused on inorganic nitrate formation, but few have focused on organic nitrates in China, precluding a thorough understanding of the nitrogen cycle and nitrate aerosol formation. Here, the inorganic (NO_3,inorg) and organic nitrate (NO_3,org) formation regimes under aerosol liquid water (ALW) and aerosol acidity (pH) influences were investigated during winter over the NCP based on data derived from an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The campaign-averaged concentration of the total nitrate was 5.3 μg m^-3, with a 13% contribution from NO_3,org, which exhibited a significantly decreased contribution with increasing haze episode evolution. The diurnal cycles of NO_3,inorg and NO_3,org were similar, with high concentrations during the nighttime at a high ALW level, revealing the important role of aqueous-phase processes. However, the correlations between the aerosol pH and NO_3,inorg (R² = 0.13, P < 0.01) and NO_3,org (R² = 0.63, P < 0.01) during polluted periods indicated a contrasting effect of aerosol pH on inorganic and organic nitrate formation. Our results provide a useful reference for smog chamber studies and promote a better understanding of organic nitrate formation via anthropogenic emissions.

Evolution of Aerosol Optical Properties from Wood Smoke in Real Atmosphere Influenced by Burning Phase and Solar Radiation

Emissions of light-absorbing black carbon (BC) and organic aerosol (OA) from biomass burning are presented as complex mixtures, which introduce challenges in modeling their absorbing properties. In this study, we chose typical residential wood burning emission and used a novel designed chamber to investigate the early stage evolution of plumes from different burning phases under real ambient conditions. The detailed mixing state between BC and OA was evaluated, on the basis of which optical modeling was performed to achieve a closure of aerosol-absorbing properties. Intensive secondary OA (SOA) formation was observed under solar radiation. OA from flaming conditions showed a higher absorptivity than from smoldering conditions, as OA is mostly internally and externally mixed with BC, respectively. For flaming (smoldering), the imaginary refractive index of OA (k_OA) was initially at 0.03 ± 0.01 (0.001) and 0.15 ± 0.02 (0.05 ± 0.02) at λ = 781 and 405 nm, respectively, with a half-decay time of 2-3 h in light but a <40% decrease under dark within 5 h. The production of less-absorbing SOA in the first 1-2 h and possible subsequent photobleaching of chromophores contributed to the decrease of k_OA. The enhanced abundance but decreased absorptivity of coatings on BC resulted in a relatively maintainable absorptivity of BC-containing particles during evolution.